As important factors of governing magnetic properties of electrical steel sheets, sizes and dispersing conditions of AlN and MnS precipitates in steels are taken up. This is why these precipitates themselves become obstacles to movements of magnetic domain walls and deteriorate not only the magnetic flux densities under a low magnetic field but also the iron loss, and in addition they hinder grain growth during recrystallization annealing, and immature grain growth thereby of ferrite grains give bad influences to developments of recrystallization texture preferable to the magnetic properties.
Coarser precipitates are preferable for grain boundary migration and magnetic domain walls during magnetization. It is important to provide the precipitations and coarsenings of AlN or MnS before the recrystallization annealing in the processes of making the electrical steel sheets.
For saving energy, it is desirable to make use of the sensible heat of the continuous cast slab by avoiding the slab temperature from going down and carrying out a hot rolling after a brief heating of the high temperature slab. However, since such a process takes an extremely short time for the slab cooling and heating in comparison with a process of reheating a cold slab as ordinarily done, a coarse precipitation of AlN taking place is not prospective.
Therefore, it has been proposed, for example, in Japanese Patent Laid-Open Nos. 108,318/77 and 41,219/79 to carry out a brief soaking before hot rolling the slab so as to coarsen the precipitation of AlN therebetween as said, and then reheat the slab to reduce a milling load of the hot rolling and to uniformalize the slab temperature for avoiding deterioration of the shape of the hot rolled sheet.
However, there arise problems, since the conventional processes require the soaking about 1000.degree. C. for more than 40 min to coarse precipitation of AlN, surface properties are deteriorated by occurrences of scales in such a period, a cover should be needed for the soaking, so that an operating efficiency is lowered by attaching and removing the cover. Further, this process could not avoid non-uniformity of the temperature due to positioning of the slab, and accordingly the magnetic properties become non-uniform by non-uniformity in coarsening of AlN precipitates.